Variable reactive coupling device



May 14,1940. A. LANDMANN 2,200,508

VARIABLE RACTIVE COUPLING DEVICE Filed April l2, 1938 l- INVENTOR 1, ALEMA/05e ANUMAN/V ATTORNEY Patented May 1.4, QL

Alexander Landmann, Ealing, London, England,

assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application april 12, 193s, sena! No. 201,536

In Great Britain May 10, 1937 c Claims. (o1. msg-44) The present invention relates to reactive coupling devices and arrangements such as transformers, more especially for use in radio systems.

The invention is primarily intended for application to Wireless receivers of the superhetero stant value notwithstanding the change in the degree of coupling between them.

The invention may be applied, for example, tocoupled tuned circuits in the intermediate frequency stage of a superheterodyne receiver; in

which, for example, inductively linked coils, one included in each of the circuits to be coupled, are wound on a former, there being provided between these co-ils a further coil which is connected in a circuit which may be opened or closed so that the 4dyne type, effective coupling between the two rst mentioned In superheterodyne receivers it is known to coils, is varied. With such an arrangement, the vary the selectivity of the receiver by varying the modifieation 'of the degree 0f Coupling by the Coupling between tuned Stages 0f the receiver, by VarlatOh Of the Circuit Of the third C011 34S above o means of a transformer coupling the two stages, described, causes avv variation in the eiective in- 10 the two coils of which are moved relatively to duetanee Of the eOulOled COlS, S0 that the varieaCh other to vary the tuning. It is also known ation in the degree of coupling causes a Variation to vary the selectivity of the receiver by varying in the tuning of the coupled CirCuitS.

a capacitive coupling between tuned stages. Both In one AeIrllOOdilrlerlt 0f the invention. tWO COilS these methods of selectivity variation also pro- 'are provided, one associated withtll'e primary and .l5 duce a variation in the reactances of the coupled One With the SeCOIlduiy Side Of the transformer, circuits and consequently of the tuning thereof, said coils being included in Circuits adapted t0 be the effect; of which can be eliminated only by shorter open-circuited according to whether the providing sensitive compensating arrangements ebOVe described third Coil iS Open 0r Short-cir- .20 which may have to be inc1udcd in circuits subcuited- The short or open-crcutine of these .2o jected to high alternating current voltage, and 'additional coils as the said third coil is open or arey in any case, expensive short-circuited,` maintains the effective induct- The object of the present invention is to proances of the linked coils in the circuits to be Vide a device for coupling a tuned circuit reac- Coupled Substantially COHSteIlt, thus maintaining .125 tively to another circuit', wherein inexpensive the tuning frequencies of the coupled `circuits sub- 25 means may be used for vai-ying the Coupling 0f stantially constant. .If desired, only one correctf the circuit and for maintaining the tuning of the ing COil may be provided. circuit constant. The invention is not only applicable to circuits A further object of the invention is to provide `Coupled thrOugh a treIlSfOlmeI', but may be ep- .,30 compensating means for maintaining the damp plied in cases Where circuits are Wholly or partly .so ing of coupled tuned circuits constant notwith- Coupled vthrough a variable caDeCitatiVe 0r Selfstanding a variation in the coupling between inductive reeetanee. them, which means are not subjected t0 high The nature of the invention and the method of alternating current voltages and may be made up carrying the invention into effect will vbe clearly .7,35 in an inexpensive manner and without using eX- understood from the following description with pensive Components, reference to the accompanying drawing in which:

According to the invention, a device for cou- Fig. 1 is a diagrammatic view of a coupling `depling a tuned circuit to another circuit is provided ViCe aeeordilg t0 the inVeIltiOIlcomprising means for varying the reactive link- Fie 2 iS a Series 0f attenuation-frequency '40 age between the circuits by a predetermined curves drawn to illustrate the operation of an ar- 40 amount, there being associated with the coupling, rangement according t0l the invention, and but not directly connected in the coupled circuits, Figs. 3 and 4 are schematic diagrams illustrata, coil 0r Coils adapted t0 be Open-circuited 0r ing tWOuIther embodiments 0f the nVentiOh. short-circuited as the case may be, to vary the Referring t0 Fig. 1, it Will be Seen that the arreactance of the tuned circuit or circuits in such rangement therein Shown comprises a coupling k5 manner as substantially to compensate for the device Cemprisiilg tWO COilS l and 2,1eSPeCtiVelY, change in the effective reactance thereof when adapted t0 be Connected in the Circuits t0 be said coupling is varied, whereby the resonant frecoupled, so that One COil aCtS aS the primary Windquency of one or both 0f the tuned circuit 0r ciring of a transformer, and the other as the seccuits may be maintained at substantially conondary winding adjacent each other on the same 50 former 3. Between the coils l and 2 is arranged a coil 4 which may be short circuited by closing switch 5. When the coil 4 is open-circuited it exerts substantially no action on the coils I and 2, but when the coiled is short-circuited it acts as a conductive screen between the coils I and 2 thus reducing the degree of coupling therebetween. The width of the pass-band on the circuits associated with coils I and 2 is thus varied by open and short-circuiting coil 4. Further, coil 4, also acts on the coils I and 2 considered separately, as well as on the degree of coupling between them, and changes the eiective inductances of these coils as regards the circuits in which they are located. Consequently, the resonant frequencies of those tuned circuits are caused to vary. In order to overcome this drawback according to the invention, coils 6 and 1 are provided arranged in circuits with switches 8 and 9 similar to the switch 5, one of the cols and I being in association with the coil I and the other being in association with the coil 2, for example, by being arranged on the same former 3 as the coils I and 2, as shown. The correcting coil 6 which is associated with the coil I is disposed on the side of the coil I away from the coil 2, and likewise the correcting coil 'l is disposed on the side of the coil 2 away from the coil I so that neither coil 6 or 1 has any substantial effect on the coupling between the coils, but `the coils are arranged so that they are linked to the coils I and 2 respectively in such a way that by open or short-circuiting the coils 6 and 'I an equal and opposite change in effective inductance of the coils I and 2 respectively is eiected to cancel the change in effective inductance of the said coils due to short or open-circuiting the circuit of the coil 4. Briey, the coils 6 and I introduce a detuning eiect equal and opposite to that inadvertently introduced by the coil 4. Preferably, switches 5, 8 and 9 are gang controlled so that coils 6 and 'I are open-circuited automatically when coil 4 is short circuited or vice versa. For example, as shown in Fig. 1, switches 5, 8 and 9 may be linked to the same operating handle I0 in such a way that when the operating handle is actuated to close switch 5, switches 8 and 9 are opened or vice versa.

The method of operation of the arrangement of Fig. l will be clearly understood from Fig. 2 in which attenuation over the coupled tuned circuits is represented vertically and frequency horizontally. The mid-frequency of the band passed by the coupled tuned circuits is represented by the vertical arrow OA in the centre of the gure. When the coil t is short-circuited, the attenuation over the circuits will be as indicated by the full-line curve 2, the point of minimum attenuation in the curve being at the midband frequency. When the coil 4 is open circuited the degree of coupling between the two circuits will be increased and the bandwidth of the circuit will be broadened and humps may tend to appear in it, in known manner, as the coupling becomes tighter. Moreover as the tuning of the circuits is modified by open-circuiting the coil 4, as above explained, the resonance curve of the receiver would become unsymmetrical about the line OA as indicated by the dotted curve I. However, by the provision of coils 6 and I according to the invention, the tuning frequencies of the coupled circuits are maintained constant notwithstanding the variation in the coupling between them and the selectivity curve with increased coupling remains symmetrical about the line OA as indicated by the chain dotted curve 22, and moreover, the damping of the circuits remains substantially unaltered. If desired, only one of the coils E and I may be provided, and by dimension of this coil, nearly the same action may be reached.

The auxiliary coils 4, Ii and l may be of single layer or multi-layer type. The switching in of the correcting coils 6 and 'I may also cause damping of the tuned circuits and thus flatten out the double hump in the characteristic produced by increase of coupling.

Fig. 3 shows an arrangement according to the invention in which the coupling between the tuned circuits comprising the coils I and 2, tuned by condensers Ia and 2a respectively, is effected wholly or partly by a top or series capacity element 25 which may be cut out of circuit by a switch such as 26 or may have an additional capacitance in parallel such as 2l, shown dotted, associated with a switch 28, so that upon actuation of the switch 2B or 28 the degree of coupling between the coupled circuits can be changed. This change in the coupling between the circuits also tends to cause a variation in the tuning frequencies of the circuits, and in order to compensate for this, arrangements including coils 6 and 'I associated with coils I and 2, as described in connection with Fig. 1 are provided.

The invention is also applicable to the case of arrangements which are coupled by a bottom condenser for example, as shown in Fig. 4, which shows an arrangement adapted to :be inserted in place of or as well as the part oi Fig. 3 included in the chain dotted rectangle V. The coupling element of Fig. 4 consists of a condenser or capacitance 3B with an element 3I in parallel which may be cut into or out of circuit by means of switch 32. The arrangement of i might be connected between the condensers la and 2a and earth instead of between the coils I and 2 as represented.

If desired, elements having substantially only selfinductance might be used instead of capacity elements in couplings treated according to the invention.

What I claim is:

1. A coupling between two circuits, at least one of which is tuned, comprising means for,

varying the reactive linkage between the circuits by a predetermined amount, a coil adapted to be open-circuited or short-circuited as the case may be, and inductively coupled to the tuned circuit to vary the reactance thereof in such manner as substantially to compensate for the change in the eliective reactance thereof when said coupling is varied, whereby the resonant frequency of the tuned circuit may be maintained at substantially constant value notwithstanding the change in the degree of coupling between the circuits.

2. A coupling arrangement-between a pair of tunable circuits comprising inductively linked coils, wound on a former, two of` which coils are inductively linked together and are adapted to be included one in each of the turnable circuits to be coupled, a third coil arranged between said two circuit coils adapted to be open or short circuited to determine the inductive linkage between said two coils, and further coils each arranged on the former adjacent to and inductively linked one with each of the two first mentioned coils, said further coils being adapted to be short or open-circuited according as said third coil is open or short-circuited as the case may be to vary the reactance of the coupled circuits in such a way that the reactances determining the tuning of the coupled circuits remain constant, notwithstanding the degree of coupling there between.

3. A coupling between two tuned circuits according to claim 1 in which the tuned circuits each including a tuning coil are Variably capacitively coupled, each of said tuning coils having inductively associated with it a coil adapted to be open-circuited or short-circuited as the case may be, to vary the reactance of the tuned circuits in such manner as substantially to cornpensate for the change in the effective reactance thereof to maintain the tuning frequency substantially constant when said coupling is varied.

4. The combination with two circuits, each having a natural resonant frequency, and a reactive coupling between said circuits, of reactive means for varying the coeilcient of coupling between said circuits, said reactive varying means tending to change the natural resonant fre quency of each circuit with variation in coupling therebetween, and means including a coil inductively coupled to at least one of said circuits and adapted to be selectively open-circuited or short-circuited for compensating for said change in natural resonant frequency.

5. The combination with a pair of inductively coupled tunable circuits, of a coil associated with the inductances of said circuits, such that with the coil short-circuited the tunable circuits are loosely coupled and with the said coil open-circuited the circuits are closely coupled, said circuits tending to change their natural resonant frequency with variation in coupling from loose to close coupling relation, and means including a coil inductively coupled to the inductance of each circuit and adapted to be selectively opencircuited or short-circuited for compensating for said change in natural resonant frequency.

6. The combination with a pair of tuned circuits having main reactances in coupled relation, of means for varying the coeiicient of coupling between said reactances, said means comprising a common auxiliary inductance which is adapted when short-circuited to provide loose coupling and when open-circuited'to provide close coupling between said circuits, said circuits tending to change their natural resonant frequency with change from loose to close coupling, separate means associated with each main reactance for compensating for said change in natural resonant frequency, said separate compensating means comprising an inductance and a switch for selectively short-circuiting and open-circuiting said inductance, and common control means for short-circuiting the separate compensating inductances simultaneously with the open-circuiting of the auxiliary coupling- Varying inductance, and vice versa.

ALEXANDER. LANDMANN. 

